What are the best ways to prevent decay on margins of crowns?

There are several components to help you prevent getting decay under crowns.1. The margins of the crowns need to be sealed.2.  Your hygiene should be immaculate in these areas including brushing regularly and flossing daily.3. Stay away from a diet that is high in sugar, carbonic acid, or citric acid (soft drinks, energy drinks, Gatorade,etc).4.  See your dentist between 2-4 times a year for professional cleanings.  5.  Have your dentist bond in all porcelain (metal free) crowns which allows for a better marginal seal and is better health around the gingival tissues.  

You mentioned that the margins of your current crowns don't fit well and are bulky. This may have been part of the problem with developing decay. If you can't clean around the crown properly you may eventually develop decay. Is the same dentist doing the new crowns that did the old ones? You may want to get a second opinion. You can also ask your dentist for a prescription strength fluoride gel to brush on your teeth. This will work better than the rinse. Also, monitor your diet and eating habits. When and what you are eating may be more important than the fact that you brush and floss.

Number one, the crowns do need to fit very well at the margins and be cleansable(that is, not bulky). Stick with your fluoride mouth rinse and perhaps have a fluoride varnish treatment whenever you have your teeth cleaned. Great job on the brushing and flossing...you are doing what you need to do, so check your diet to avoid sugar and acidic drinks. Are you taking any medications that are making your mouth dry? A lack of sailva can lead to decay very quickly. Talk to your dentist about all of these issues and concerns.